Speech coder output transformation method for reducing audible noise

Abstract

In a cellular telephone system where a digital cellular telephone is connected to a regular telephone through the public switched telephone network (PSTN), a speech encoder/decoder is used with an A/μ-Law encoder/decoder causing annoying audible noise at very low levels because of the quantization characteristics of the A/μ-Law encoder/decoder. This noise is eliminated by adding a digital constant to the output of the speech coder, shifting the low level signal away from zero. The resulting DC level added to the speech signal is inaudible to the PSTN telephone user and does not degrade speech quality. Alternatively, the constant added to the output of the speech coder is confined to a small value added to the speech coder output to move the entire speech coder output during the silence period, between speech periods, above zero or below zero.

What is claimed is:  
     
       1. In a speech communication system having a speech decoder followed by a compander, the speech decoder having an output signal with a plurality of output values, a method for reducing audible noise generated by the compander during the absence of a voice signal, the steps of the method comprising: 
       selecting a range of output values from the plurality of output values of the output signal, wherein the range of output values is defined by the output values of less than zero and greater than or equal to −2;  
       calculating each output value of the plurality of output values;  
       determining if an output value of the plurality of output values falls within the range;  
       adding a predetermined constant value to the output signal if the determining determines that the output value of the plurality of output values falls within the range; and  
       supplying the output signal to the compander.  
     
     
       2. The method of  claim 1  wherein the constant value added to the output signal of the decoder is a +2 value that moves all the −1 and −2 portions of the speech decoder output signal to a positive level or zero level. 
     
     
       3. The method of  claim 1  wherein the compander is an A-Law converter. 
     
     
       4. The method of  claim 1  wherein the compander is a μ-Law converter. 
     
     
       5. In a speech communication system having a speech decoder followed by a compander, the speech decoder having an output signal with a plurality of output values, a method for reducing audible noise generated by the compander during the absence of a voice signal, the steps of the method comprising: 
       selecting a range of output values from the plurality of output values of the output signal, wherein the range of output values is defined by the output values of greater than zero and less than or equal to +2;  
       calculating each output value of the plurality of output values;  
       determining if an output value of the plurality of output values falls within the range;  
       adding a predetermined constant value to the output signal if the determining determines that the output value of the plurality of output values falls within the range; and  
       supplying the output signal to the compander.  
     
     
       6. The method of  claim 5  wherein the constant value added to the output signal of the decoder is a −2 value that moves all the +1 and +2 portions of the speech decoder output signal to a negative level or zero level. 
     
     
       7. The method of  claim 1  wherein the compander is an A-Law converter. 
     
     
       8. The method of  claim 1  wherein the compander is a μ-Law converter. 
     
     
       9. In a speech communication system having a speech decoder followed by a compander, the speech decoder configured to cause a reduction in audible noise generated by the compander during the absence of a voice signal, the speech decoder comprising: 
       an output signal with a plurality of output values;  
       a controller configured to select a range of output values from the plurality of output values of the output signal, wherein the range of output values is defined by the output values of less than zero and greater than or equal to −2, the controller further configured to calculate each output value of the plurality of output values and determine if an output value of the plurality of output values falls within the range;  
       an adder configured to add a predetermined constant value to the output signal if the controller determines that the output value of the plurality of output values falls within the range; and  
       an output supplier configured to supply the output signal to the compander.  
     
     
       10. The method of  claim 9  wherein the compander is an A-Law converter. 
     
     
       11. The method of  claim 9  wherein the compander is a μ-Law converter. 
     
     
       12. In a speech communication system having a speech decoder followed by a compander, the speech decoder configured to cause a reduction in audible noise generated by the compander during the absence of a voice signal, the speech decoder comprising: 
       an output signal with a plurality of output values;  
       a controller configured to select a range of output values from the plurality of output values of the output signal, wherein the range of output values is defined by the output values of greater than zero and less than or equal to +2, the controller further configured to calculate each output value of the plurality of output values and determine if an output value of the plurality of output values falls within the range;  
       an adder configured to add a predetermined constant value to the output signal if the controller determines that the output value of the plurality of output values falls within the range; and  
       an output supplier configured to supply the output signal to the compander.  
     
     
       13. The method of  claim 12  wherein the compander is an A-Law converter. 
     
     
       14. The method of  claim 12  wherein the compander is a μ-Law converter. 
     
     
       15. The speech decoder of  claim 9 , wherein the constant values added to the output signal of the decoder is a +2 value that moves all the −1 and −2 portions of the speech decoder output signal to a positive level or zero level. 
     
     
       16. The speech decoder of  claim 12 , wherein the constant values added to the output signal of the decoder is a −2 value that moves all the +1 and +2 portions of the speech decoder output signal to a negative level or zero level.